Low power consumption of integrated circuits is an important feature of modern technology products of compact size, high mobility, and green energy. However, multiple objectives increase the complexity and scale of integrated circuits (ICs), which easily results in high power consumption. Using multiple voltage levels is therefore a popular method to lower the power consumption of ICs. Level shifters play an important role for signal integrity among various voltage levels in these multiple-voltage IC designs, and wide-voltage-range level shifters provide a high flexibility in supply-voltage assignment, by which the circuit performance and power consumption can be adjusted. Currently, a wide voltage range can cover conventional super-threshold supply voltages and sub-threshold supply voltages for optimizing the power consumption of ICs, and the operating range of level shifters is required to expand accordingly.
FIG. 1 shows a schematic drawing of a level shifter 100. I1 is a low-voltage inverter. This level shifter is able to convert a sub-threshold input signal IN to a super-threshold output signal OUT. The level shifter 100 utilizes a two-stage structure of conventional cross-coupled level shifters. The driving strength of the P-type transistors is reduced in the first stage, where the diode clamps and drops the drain-to-source voltage of the P-type transistors. However, the level shifter 100 requires dual-threshold transistors, including standard-threshold and low-threshold transistors, to convert a sub-threshold input signal toward a super-threshold output signal. Moreover, the level shifter 100 needs to be adapted to the required supply-voltage ranges, otherwise the delay time of the level shifter 100 is very slow in some combinations of input-to-output voltages.
FIG. 2 shows a level shifter 200. The level shifter 200 utilizes a Wilson current mirror for stably converting a sub-threshold input signal IN3 to a super-threshold output signal OUT3. The level shifter 200 needs only the standard-threshold transistors. However, the level shifter 200 becomes very slow when the voltage difference is slight between the input and output levels, in which case the rising delay is much greater than the falling delay and not practical for IC application.
FIG. 3 shows a schematic drawing of another level shifter 300. The level shifter 300 utilizes a pair of cross-coupled NOR gates, where two N-type transistors, M16 and M17, are in charge of differential sensing. An inverted input signal is fed to the N-type transistors M18 to accelerate the speed of the level shifter. However, the available voltage of the input signal is confined to a near-threshold value due to the cross-coupled structure at the first stage.
A high-speed and wide-voltage-range lever shifter is urgent and critical for the IC industry, although it is very difficult that the operating range covers both sub-threshold and super-threshold voltages. In addition, bidirectional level conversion is valuable for multiple voltage ICs; bidirectional level conversion indicating that the level conversion can be performed from a low voltage to a high voltage and from a high voltage to a low voltage. Furthermore, it is preferred that the operating voltage range of the lever shifter is insensitive to the manufacturing technology, process variations, and operation environments.